A capacitive microphone typically includes a diaphragm including an electrode attached to a flexible member and a backplate parallel to the flexible member attached to another electrode. The backplate is relatively rigid and typically includes a plurality of holes to allow air to move between the backplate and the flexible member. The backplate and flexible member form the parallel plates of a capacitor. Acoustic pressure on the flexible member causes it to deflect which changes the capacitance of the capacitor. The change in capacitance is processed by electronic circuitry to provide an electrical signal that corresponds to the change.
Microelectromechanical systems (MEMS), including miniature microphones, are fabricated with techniques commonly used for making integrated circuits. Potential uses for MEMS microphones include microphones for hearing aids and mobile telephones, and pressure sensors for vehicles.
Once a silicon microphone has been fabricated it must be packaged onto a device. During this packaging process the backplate of the silicon microphone may displace or deform. Any movement of the backplate during packaging may reduce the sensitivity of the microphone or prevent operation of the microphone.
Factors that limit the performance of a silicon microphone include leakage currents between the two parallel plates of the capacitor, parasitic capacitances in the microphone, and stiction between the diaphragm and the backplate.
Leakage currents occur when the two plates of the microphone capacitor are not completely isolated from each other. Compromised isolation may occur when providing bond pads to the microphone during dicing of the wafers. Leakage currents reduce the impedance of the silicon microphone. Ideally the impedance should be infinite; however, there will always be some leakage due to microphone processing and design. In some systems the problems of leakage currents are overcome by using a charge pump in a pre-amp. The use of a pre-amp and charge pump allows the microphone to be run at a voltage greater than the desired operating voltage and is less sensitive to leakage currents.
Parasitic capacitances can be caused by debris that reside on the edge of a wafer after dicing. Parasitic capacitances are stray capacitances that are generated due to unwanted influences such as dielectric layers. These capacitances also affect the performance of the silicon microphone.
Stiction is a common problem for small capacitive devices. One area where stiction may occur is during dicing of the microphone wafer. Typically the microphone wafer is protected by placing some adhesive tape on the top side to protect the thin diaphragm. At the same time the wafer also sits on another piece of adhesive tape so that water will not enter the back side of the wafer during dicing. These two protection tapes form an enclosed air column between the top diaphragm and the bottom wafer. As the diaphragm is a thin membrane, any temperature change may expand the enclosed air column thereby pushing or pressurising the diaphragm. This pressure may cause the diaphragm to touch the backplate which is only a few microns away. After a small contact time between the backplate and the diaphragm there will be a bond formed and hence stiction has occurred.